Load Calculation of Psychrometry MCQ Quiz - Objective Question with Answer for Load Calculation of Psychrometry - Download Free PDF

Explanation:

Definition of Sensible Heat Factor (SHF):

• The sensible heat factor (SHF) is a term commonly used in the field of thermal engineering, particularly in heating, ventilation, and air conditioning (HVAC) systems. It is defined as the ratio of the sensible heat to the total heat of the system. This factor is crucial in determining the performance and efficiency of HVAC systems, as it helps in understanding the proportion of heat used for changing the temperature of the air compared to the total heat added or removed.

Mathematical Representation:

The sensible heat factor can be mathematically represented as follows:

SHF = Sensible Heat / Total Heat

Where:

• Sensible Heat (Q_s): The heat added or removed to change the temperature of the air without changing its moisture content.
• Total Heat (Q_t): The sum of sensible heat and latent heat, where latent heat is the heat required to change the moisture content of the air without changing its temperature.

Applications:

Understanding the SHF is essential in various applications, including:

• Air Conditioning Systems: To ensure comfort by maintaining the desired temperature and humidity levels.
• Heating Systems: To efficiently heat spaces by primarily focusing on temperature control.
• Industrial Processes: To maintain optimal conditions for processes that are sensitive to temperature and humidity variations.

Explanation:

Air conditioning

• Air conditioning is the simultaneous control of the temperature of the air, the velocity of air, the purity of air, and humidity of the air.
• It is the process of altering the properties of air to more favorable conditions.
• Air conditioning refers to any form of technological cooling, heating, ventilation, and disinfection that modifies the condition of air.

Applications of air conditioning

• It is used in houses, hospitals, theatres, offices, and in computers.
• Air conditioning of transport media such as buses, cars, trains, airplanes, and ships.
• Wide application in food processing, cold storage, printing, chemicals, and pharmaceutical and machine tools, etc.

Additional Information 

Concept:

There are two methods of estimating the infiltration of air into conditioned space due to wind action. They are

 i) Crack method       ii) Air change method

In the crack method, the estimation is based on measured leakage characteristics and width and length of cracks (perimeter) around windows or doors.

The air change method assumes a certain number of air changes per hour for each space depending on its usage.

Infiltration = 1 air change per hour

cmm = volumetric flow rate cubic meter per minute 

Sensible heat load due to infiltration,

SHL =[ 0.0204 x cmm x ∆T] kW

Calculation:

Given:

Infiltration = 1 air change per hour

cmm = volumetric flow rate cubic meter per minute = (20x30x4)/60 = 40m³/min

∆T = (40^o – 25^o) = 15^oC

Sensible heat load due to infiltration,

SHL =[ 0.0204 x cmm x ∆T] kW

= 0.0204 x 40 x 15 = 12.24 KJ/s = 12.24 kW

Correct option is (b), i.e. 12.24kW

Concept:

The sensible heat factor is the ratio of Sensible heat to the Room total heat.

Room total heat = Sensible heat (SH) + Latent heat (LH)

Sensible Heat Factor is given by, 

\(SHF = \frac{{SH}}{{LH + SH}} \)

Calculation:

Given, SH = 3, LH = 1

\(SHF = \frac{{SH}}{{SH + LH}} = \frac{{3}}{{3+1}} = 0.75\)

Explanation:

The sensible heat factor is the ratio of Sensible heat to the Room total heat.

Room total heat = Sensible heat (SH) + Latent heat (LH)

Sensible Heat Factor is given by, 

\(\rm SHF = \frac{{SH}}{{LH \;+\; SH}} \)

From the ratio we can conclude that if both latent heat and sensible heat is either negative or positive, the sensible heat factor will be positive, thus statements 2 and 3 are correct answer.

Concept:

The sensible heat factor is the ratio of Sensible heat to the Room total heat.

Room total heat = Sensible heat (SH) + Latent heat (LH)

Sensible Heat Factor is given by, 

\(SHF = \frac{{SH}}{{LH + SH}} \)

Calculation:

Given:

LH = 0.25 SH

\(SHF = \frac{{SH}}{{SH + LH}} = \frac{{SH}}{{SH + \;0.25\;SH}} = 0.8\)

Concept:

For saturated air relative humidity (ϕ) is 100% i.e. P_v = P_vs

i.e. partial pressure of water vapour in the moist air is equal to the saturation pressure of the water vapour

Specific Humidity: It is the ratio of the mass of water vapour to the mass of air at a given temperature and volume.

Specific humidity \(\omega = \frac{{{m_v}}}{{{m_a}}} \times \frac{{{P_v}}}{{{P_t} - {P_v}}}\)

where P_v is the partial pressure of water vapour and P_t is the total pressure of moist air.

Calculation:

If we find the specific humidity before the cooling coil and after the cooling coil then the difference between two will give the amount of water condensed in the cooling coil.

Before entering into the cooling coil,

ϕ = 80% = 0.8, m_v = 18 g/mol, m_a = 28.94 g/mol, P_t = 105 kPa

At 30°C P_vs = 4.24 kPa

Relative humidity (ϕ) \(= \frac{{{P_v}}}{{{P_{vs}}}}\)

∴ P_v = ϕ × P_vs = 0.8 × 4.24

⇒ P_v = 3.392 kPa

Specific humidity (ω) \(= \frac{{{m_v}}}{{{m_a}}} \times \frac{{{P_v}}}{{{P_t} - {P_v}}}\)

\({\left( \omega \right)_{moistair}} = \frac{{18}}{{28.94}} \times \frac{{{P_v}}}{{105 - {P_v}}}\)

\(\therefore {\omega _{moistair}} = \frac{{18}}{{28.94}} \times \frac{{3.392}}{{105 - 3.392}} = \;0.0207{\rm{\;kg}}/{\rm{kg\;of\;dry\;air}}\)

Now after the exit of the cooling coil

At 15°C P_vs = 1.7 kPa

For saturated air ϕ = 1

⇒ P_v = P_vs = 1.7 kPa

\({\omega _{saturated\;air}} = \frac{{18}}{{28.94}} \times \frac{{1.7}}{{100 - 1.7}} = \;0.0107{\rm{\;kg}}/{\rm{kg\;of\;dry\;air}}\)

∴ Mass of water condensing = ω_{moist air} – ω_{saturated air}

⇒ Mass of water condensing = 0.0207 – 0.0107 = 0.010 kg/kg of dry air

Mass of water condensing out from duct = 10 gm/kg of dry air

Explanation:

Humidity Ratio at inlet (ω₁) = 19 g/kg of dry air = 19 × 10^-3 kg/kg of dry air

Humidity Ratio at outlet (ω₂) = 8 × 10^-3 kg/kg of dry air

Mass of water vapour at inlet m_vi = ω₁ × m_a

 = 19 × 10^-3 × 3 kg/s

= 57 × 10^-3 kg/s

Mass of water vapour at outer m_vo = ω₂ × m_a

= 8 × 10^-3 × 3

= 24 × 10^-3 kg/s

Mass of water that condensed = m_vi - m_vo

= (57 - 24) × 10^-3

= 33 × 10^-3 kg/s

= 0.033 kg/s

Cooling capacity of coil = h_i - h₀ - h_c

Where h_i = Inlet enthalpy, h₀ = outlet enthalpy, h_c = enthalpy of condensate;

A = [(85 - 43) kJ/kg × 3 kg/s] - [67 kg/kg × 0.033]

Explanation:

Air conditioning

• Air conditioning is the simultaneous control of the temperature of the air, the velocity of air, the purity of air, and humidity of the air.
• It is the process of altering the properties of air to more favorable conditions.
• Air conditioning refers to any form of technological cooling, heating, ventilation, and disinfection that modifies the condition of air.

Applications of air conditioning

• It is used in houses, hospitals, theatres, offices, and in computers.
• Air conditioning of transport media such as buses, cars, trains, airplanes, and ships.
• Wide application in food processing, cold storage, printing, chemicals, and pharmaceutical and machine tools, etc.

Additional Information 

Concept:

SHF (Sensible Heat Factor):

It is defined as a sensible heat load divided by total heat load.

\({{S}}.{{H}}.{{F}} = \frac{{{{S}}.{{H}}}}{{{{T}}.{{H}}}}=\frac{S.H}{S.H\;+\;L.H}\)

where T.H = S.H + L.H

S.H = Sensible heat T.H = Total heat L.H ⇒ Latent heat 

Calculation:

Given:

S.H = 30000 kJIhr L.H = 20000 kJIhr.

\({{S}}.{{H}}.{{F}} =\frac{S.H}{S.H\;+\;L.H}\)

\({\rm{S}}.{\rm{H}}.{\rm{F}} = \frac{{30,000}}{{30,000 + 20,000}} = \frac{3}{5} = 0.60\;\)

Human Comfort and Effective temperature: Human comfort in the air conditioning can be defined as comfort which is experienced by a person through adjustment of dry bulb temperature, relative humidity and air velocity.

The effective temperature is the temperature of the saturated environment at which motionless saturated air would induce same level of comfort as experienced in a normal environment in a sedentary worker wearing ordinary indoor clothing.

From the above definition we can say that Statement I) is correct.

Concept:

Cooling Loads Classified by Kinds of Heat:

There are two distinct components of the air conditioning load;

(1) The sensible load (heat gain) and

(2) The latent load (water vapour gain)

Sensible Loads:

Sensible heat gain is the direct addition of heat to a space, which shall result in increase in space temperatures.

The factors influencing sensible cooling load:

1) Solar heat gain through building envelope (exterior walls, glazing, skylights, roof, floors over crawl space)

2) Partitions (that separate spaces of different temperatures)

3) Ventilation air and air infiltration through cracks in the building, doors, and windows

4) People in the building

5) Equipment and appliances operated in the summer (fan, Cooler, AC)

6) Lights

Latent Loads:

A latent heat gain is the heat contained in water vapour. Latent heat does not cause a temperature rise, but it constitutes a load on the cooling equipment. Latent load is the heat that must be removed to condense the moisture out of the air.

The sources of latent heat gain are:

1) People (breathing)

2) Cooking equipment

3) Housekeeping, floor washing etc.

4) Appliances or machinery that evaporates water

5) Ventilation air and air infiltration through cracks in the building, doors, and windows

Total Cooling load = Sensible Cooling load + Latent Cooling load

Concept:

Sensible heat factor: it is the ratio of sensible heat transfer to the total heat transfer.

Sensible heat (SH): It is the amount of heat required to change the temperature by 1^∘C 

Latent heat (LH): it is the amount of heat required to change the phase of the substance at a constant temperature.

Total heat: Sensible heat (SH) + Latent heat (LH)

Sensible heat factor: \(SHF = \frac{{SH}}{{LH + SH}} \)

Calculation:

Given:

SH = 40 kJ/s, LH = 30 kJ/s

\(SHF = \frac{{SH}}{{LH + SH}} \)

\(SHF = \frac{{40}}{{30 + 40}} = \frac{{40}}{{70}}\) = 0.57

Explanation:

The sensible heat factor is the ratio of Sensible heat to the Room total heat.

Room total heat = Sensible heat (SH) + Latent heat (LH)

Sensible Heat Factor is given by, 

\(\rm SHF = \frac{{SH}}{{LH \;+\; SH}} \)

From the ratio we can conclude that if both latent heat and sensible heat is either negative or positive, the sensible heat factor will be positive, thus statements 2 and 3 are correct answer.

Explanation:

Effective temperature 

• It is the temperature of saturated air at which a person or human being would feel the same level of comfort as in the actual environment.
• It includes comfort temperature, velocity, and purity.

Factors affecting the effective temperature

Climatic and seasonal differences

• People living in a warmer region feel comfortable at higher effective temperatures than people living in a colder region.
• In summer the effective temperature is 21.6°C and in the case of winter, the optimum effective temperature is 20°C.

Age and Gender

• Children and old age persons need 2-3°C higher effective temperature in comparison to adults. Same the case is with the women.

Kind of activity

• If a person is involved in the foundry shop, dancing or jogging, etc needs a lower effective temperature in comparison to the person in the rest.

Density of occupants

• Highly densely occupied needs a lower effective temperature in comparison to the less densely occupied.

Comfort chart

Figure: Comfort chart

• ASHRAE (American Society of Heating Refrigeration and  Air conditioning Engineers), conducted a survey on different kinds of people subjected to a wide range of environmental temperature conditions, humidity purity, and air velocity and developed the above comfort chart. 
• If the value of relative humidity is above 60%, then the tendency of sticky sensation develops and if the value of relative humidity is less than 50%, then the skin surface is too dry. So the relative humidity for comfort air conditioning is 60%.